1. Field of the Invention
The present invention relates to a coating deposition method based upon colloidal processing technology.
2. Description of Related Art
A coating layer on a substrate, such as a ceramic film (i.e., coating) deposited on a metal or oxide substrate, can be obtained by several methods. Generally such films can be deposited using methods either requiring or not requiring vacuum technology.
Contemporary vacuum deposition techniques can be grouped into two categories: physical vapor deposition (such as sputtering, laser ablation, etc.) and chemical vapor deposition. Both technologies require expensive vacuum pumping equipment. Because of the relatively high cost of capital equipment, such methods are usually not economically viable for high volume applications.
Physical vacuum deposition methods are also limited because the are xe2x80x9cline-of-sight.xe2x80x9d That is, deposition only occurs on the surface of the substrate which can be xe2x80x9cseenxe2x80x9d by the source. Substrates having a more complex geometry than planar typically are poorly coated, if at all, in a vacuum deposition system. Complex geometrical substrates may be rotated and turned in a vacuum system to achieve more complete surface coverage, although this adds considerable complexity to the system. Chemical vapor deposition is more conformal; however, it often uses toxic and/or expensive chemical reactants. Both physical and chemical deposition techniques generally have low deposition rates for oxide films, typically less than 1 micron per hour.
Contemporary non-vacuum methods of applying coatings to substrates include plasma spraying, tape casting; tape calendering; screen printing; sol-gel coating; colloidal spin or dip coating; electrophoretic deposition; slurry painting; and spray pyrolysis coating. Tape casting and tape calendering are generally limited to planar substrates only. Plasma spraying, slurry painting, and screen printing techniques usually yield coatings with almost certain porosity and are thus more appropriate for applications where a fully dense film is not required. Spray pyrolysis, in which a solution of metal salts or organometallics is sprayed on a heated substrate also generally yields porous films.
Colloidal techniques (spin coating, dip coating, and electrophoretic deposition) are among the most cost-effective techniques known for deposition of dense thin films. These techniques involve the preparation of a colloidal solution of the ceramic powder of the material to be coated. In the spin coating method, a few drops of the colloidal solution is placed on the surface of the substrate, which is subsequently spun at high speed thereby removing the solvent and leaving a thin layer of the powder on the surface of the substrate. This technique is limited to deposition onto planar substrates having low surface areas.
In electrophoretic deposition, a high voltage is applied between the substrate and a counter electrode, both of which are immersed in the colloidal suspension. The powder particles, which are generally slightly charged on the surface, move under the electrostatic potential toward the substrate where they discharge and deposit. This technique is limited to conductive substrates only.
In the dip coating process, the substrate is dipped into the colloidal solution followed by withdrawal and drying. During the air-drying step, the solvent evaporates, leaving the powder in the form of a thin film on the substrate.
It has been extremely difficult, if not impossible, to deposit coatings with thicknesses larger than a few microns, using conventional dip coating methods. The films obtained are generally limited in thickness, typically a few microns, but less than ten microns. Attempts to deposit thicker coatings have not generally been successful because of film cracking, particularly during the drying process. The drying step in a conventional colloidal dip coating process is done after withdrawing the substrate from the solution. During the drying step the solvent evaporates which induces film shrinkage due to a large volume change which in turn leads to cracking. In order to deposit coatings thicker than 10 microns, the coating process must be repeated, which is both time consuming and costly.
In addition, all the colloidal processing techniques require subsequent sintering at high temperature in order to densify the film. The process of thermal cycling of the substrate from room temperature to the sintering temperature, can cause cracking between the successive layers because of differential rates of thermal expansion.
Accordingly, a need exists for coatings on substrates that can be relatively dense, are essentially crack-free, yet sufficiently thick (i.e., greater than 10 xcexcm), and preparable in a single dispersion step.
It is an object of the present invention to produce dense coatings on various substrates.
A further object of the invention is to provide coatings on various substrates in a single processing step.
Another object of the invention is to provide a dense or porous coating on a substrate.
Another object of the invention is to provide coatings of single phase materials or a composite of various materials such as oxide, nitride, silicide, and carbide compounds.
Another object of the invention is to provide coatings at low cost compared to conventional thin film deposition techniques.
Another object of the invention is to provide coatings prepared by spraying with an ultrasonic atomizer.
Another object of the invention is to provide coatings of two or more materials with a graded composition through at least one portion of the coating.
Another object of the invention is to provide coatings on substrates that substantially reduce the stress at the interface between coating and substrate.
The present invention is a new colloidal coating deposition method that can produce dense (i.e., greater than about 90% of theoretical density), crack-free coatings at virtually any thickness ranging from less than one micron to several hundred microns in a single deposition step. The present invention includes the preparation of a stable colloidal solution containing a powder of the material to be coated and a carrier medium (e.g., solvent) prior to deposition. Subsequently, the colloidal solution (e.g., colloidal suspension) is then sprayed on the substrate to be coated, using a spraying device, preferably an ultrasonic nebulizer. The substrate is heated to a temperature higher than the boiling point temperature of the solvent, which hastens evaporation of the solvent, leaving the powder in the form of a compact coating layer. Deposition of the coating onto a heated substrate is critical to the formation of a thick coating without cracks. Also, a fine and uniform spray obtained using ultrasonic nozzles is an important feature in the formation of high quality coatings.
To facilitate solvent evaporation, the solvent used in the subject invention is preferably chosen from among those having sufficiently high volatility. When water must be used, an organic solvent is often added to increase solvent volatility and enhance surface wetting properties. The method of the invention can be termed Colloidal Spray Deposition (CSD). CSD allows the deposition of thin, thick, or complex coatings that have generally been unattainable heretofore. Using the present method, a coating several microns to several hundred microns in thickness can easily be prepared using a single step. The coating can encompass a dense, or porous sintered particle layer that matches the desired application. By controlling the composition of the colloidal solution delivered to an ultrasonic nozzle, coatings with either simple or complex structures can be created, such as composites of different materials or coatings with graded compositions, including continuously graded or discontinuously graded, including stepped compositions. For example, by controlling the feed rates of the colloidal solutions into the nozzle for each of the constituent particle sources, the concentration of the ceramic composites may be continuously graded from one (or more) composition(s) to another.
An advantage of the invention is that it provides coatings for several applications, including solid oxide fuel cells, gas turbine blade coatings, sensors, surface catalyst coatings, steam electrolyzers, and in any application where an chemically inert protective coating of oxide, silicide, nitride or carbide material is desired.